Fringe capacitor using bootstrapped non-metal layer

ABSTRACT

Capacitors configured in a switched-capacitor circuit on a semiconductor device may comprise very accurately matched, high capacitance density metal-to-metal capacitors, using top-plate-to-bottom-plate fringe-capacitance for obtaining the desired capacitance values. A polysilicon plate may be inserted below the bottom metal layer as a shield, and bootstrapped to the top plate of each capacitor in order to minimize and/or eliminate the parasitic top-plate-to-substrate capacitance. This may free up the bottom metal layer to be used in forming additional fringe-capacitance, thereby increasing capacitance density. By forming each capacitance solely based on fringe-capacitance from the top plate to the bottom plate, no parallel-plate-capacitance is used, which may reduce capacitor mismatch. Parasitic bottom plate capacitance to the substrate may also be eliminated, with only a small capacitance to the bootstrapped polysilicon plate remaining. The capacitors may be bootstrapped by coupling the top plate of each capacitor to a respective one of the differential inputs of an amplifier comprised in the switched-capacitor circuit.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates generally to the field of semiconductor circuitdesign, and more particularly to the design of a capacitor structure ona semiconductor substrate.

2. Description of the Related Art

Many integrated circuits (ICs), including mixed-signal circuits thatinclude both digital and analog components, oftentimes requirehigh-performance capacitors configured on the chip. Currently, a widevariety of applications for example dynamic random access memories,phase-locked loops, voltage controlled oscillators, operationalamplifiers, and switched capacitor circuits—feature capacitors formed onintegrated circuits. Generally, these on-chip capacitors can also beused to decouple digital and analog integrated circuits from potentialnoise that may be generated by the rest of the system. In many of thepresent systems, on-chip capacitors are designed as metal-to-metalcapacitors due to the advantages such capacitors typically have overother types of capacitors, for example over capacitors formed from gateoxide. For example, in order to avoid costs associated with ametal-insulator-metal capacitor—such as additional masks and additionalwafer processing costs—, it is generally desired to form a capacitorusing the multiple layers of routing metal available in any givenprocess.

Metal-to-metal capacitor structures are typically stable, predictable,and provide high-capacitance and low on-chip leakage. Metal-to-metalcapacitors also provide better linearity than gate-oxide capacitors, andthe quality factor of metal-to-metal capacitors is generally independentof the DC voltage of the capacitor. Such structures, however, oftentimesconsume a large area of the IC. In order to reduce the required area,capacitors are many times fabricated as parallel-plate capacitorstructures using two or more layers of routing metal in the IC.Accordingly, the capacitors are often designed using multiple layers ofstacked, alternately connected metal, which form the opposing electricalnodes of the capacitor.

However, in small geometry processes, the fringe-capacitance betweenmetal lines within the same metal layer can be large, and offers analternate method for constructing metal-to-metal capacitors. It isgenerally possible to control the spacing between the metal lines withinthe same metal layer through accurate lithography. In contrast, thecapacitance between the various metal layers may not be as effectivelycontrolled, since the thickness of the field-oxide region in thecorresponding metal-‘field-oxide’-metal structure can generally varyfrom lot to lot and across a die/wafer. Thus, using thefringe-capacitance between metal lines within the same metal layer toconstruct metal-to-metal capacitors offers notable advantages.

For example, in switched capacitor circuits, it has generally beendesirable to design a well matched capacitor in order to obtain highaccuracy. Typically, the goal has been to maximize capacitive density inorder to minimize the die area occupied by the capacitor, and tominimize the ‘top-plate’-to-substrate capacitance in order to avoidelectric charge being drained from critical nodes of the system throughparasitic capacitance. In other words, it is oftentimes desirable tocreate very accurately matched, high capacitance density capacitorswithout paying the additional cost of parallel-plate capacitors that maybe available in a given fabrication process. A minimal‘top-plate’-to-substrate capacitance is preferable because such acapacitance can be a source of errors in switched capacitor circuits. Inmost current fringe-capacitance solutions, the top plate is shieldedusing the metal layer closest to the substrate (or bottom metal layer),hence eliminating the ‘top-plate’-to-substrate capacitance. This,however, reduces the capacitance density of the fringe-capacitance,since the bottom metal layer cannot be used when forming the desiredfringe-capacitance.

Many other problems and disadvantages of the prior art will becomeapparent to one skilled in the art after comparing such prior art withthe present invention as described herein.

SUMMARY OF THE INVENTION

In one set of embodiments, accurate high density capacitors may beconstructed on an integrated circuit by using only fringe-capacitancedeveloped between metal lines within a given metal layer and minimizingor completely eliminating parallel-plate-capacitance. In order tomaximize fringe-capacitance, the metal lines comprising the top andbottom plates of the capacitor may be interdigitated with minimumspacing, and parallel-plate-capacitance may be minimized or eliminatedby stacking top plate traces on top of each other and bottom platetraces on top of each other. Therefore, a top level layer in amulti-layer process may be used for routing, and all layers below thetop level layer, including the bottom layer, may be configured asinterdigitated structures to maximize capacitance density. In order tominimize capacitance developed between the top plate in the bottom metallayer and the substrate, a low-impedance conductive plate constructed inpolysilicon layer may be inserted between the bottom metal layer and thesubstrate. In one embodiment, the polysilicon plate is bootstrapped tothe top plate to drive the polysilicon-to-substrate capacitance andminimize or eliminate any charge transfer from the top plate to thepolysilicon plate. Bootstrapping the polysilicon plate to the top platemay also minimize and/or eliminate the bottom-plate-to-substratecapacitance, and all metal layers that are not used for routing may beused in constructing the capacitor(s).

In one embodiment, the bootstrapping of the polysilicon plate to the topplate of the capacitor may be implemented by coupling the top plate ofthe capacitor to the gate terminal of an NMOS device, and coupling thepolysilicon plate to the source terminal of the NMOS device. As aresult, the voltage at the polysilicon plate may track the voltage atthe top plate, with no considerable voltage change across any parasiticcapacitance that may have formed from the top plate of the capacitor tothe polysilicon plate. By minimizing or eliminating current flow fromthe top plate of the capacitor to the polysilicon plate during circuitoperation, the parasitic top-plate-to-polysilicon-plate capacitance mayeffectively be removed from a circuit comprising metal-to-metalcapacitors. In one set of embodiments, a switched-capacitor circuit maybe configured in an integrated circuit, with the capacitors of theswitched-capacitor circuit configured as metal-to-metal capacitors usingfringe-capacitance, with a polysilicon plate configured between thebottom metal layer and the substrate. The top plate of each capacitor ofthe switched-capacitor circuit may be configured to couple to acorresponding differential input terminal of the differential inputstage of an amplifier of the switched-capacitor circuit. Thedifferential input stage may comprise a pair of PMOS devices, which mayhave their respective source terminals coupled to the gate terminal ofan NMOS device, with the source terminal of the NMOS device coupled tothe polysilicon plate. The NMOS device may follow the common mode inputof the amplifier, and drive the polysilicon plate without affecting theperformance of the amplifier or the capacitance at the top plate of eachcapacitor.

Other aspects of the present invention will become apparent withreference to the drawings and detailed description of the drawings thatfollow.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing, as well as other objects, features, and advantages ofthis invention may be more completely understood by reference to thefollowing detailed description when read together with the accompanyingdrawings in which:

FIG. 1 is a diagram illustrating the electric fields between topcapacitor plates and bottom capacitor plates for one embodiment of ametal-to-metal capacitor structure, showing two metal layers;

FIG. 2 is a diagram illustrating the electric fields between topcapacitor plates and bottom capacitor plates of one embodiment of ametal-to-metal capacitor structure in a bottom metal layer, and theelectric fields between the entire bottom metal layer and the substrate;

FIG. 3 is a diagram illustrating the electric fields for themetal-to-metal capacitor structure shown in FIG. 2, when a low-impedancepolysilicon plate is inserted between the bottom metal layer and thesubstrate;

FIG. 4 is a circuit diagram of one embodiment of a bootstrappingconfiguration for bootstrapping the polysilicon plate inserted betweenthe bottom metal layer and the substrate, to the top capacitor plate;

FIG. 5 is one embodiment of a switched-capacitor circuit that can beconfigured with metal-to-metal capacitors having a polysilicon plateinserted between the bottom metal layer and the substrate, with thepolysilicon plate bootstrapped to the top capacitor plates; and

FIG. 6 is a circuit diagram of one embodiment of the bootstrappingconfiguration of FIG. 4 applied to the input stage of the amplifiercomprised in the switched-capacitor circuit of FIG. 5.

While the invention is susceptible to various modifications andalternative forms, specific embodiments thereof are shown by way ofexample in the drawings and will herein be described in detail. Itshould be understood, however, that the drawings and detaileddescription thereto are not intended to limit the invention to theparticular form disclosed, but on the contrary, the intention is tocover all modifications, equivalents, and alternatives falling withinthe spirit and scope of the present invention as defined by the appendedclaims. Note, the headings are for organizational purposes only and arenot meant to be used to limit or interpret the description or claims.Furthermore, note that the word “may” is used throughout thisapplication in a permissive sense (i.e., having the potential to, beingable to), not a mandatory sense (i.e., must).” The term “include”, andderivations thereof, mean “including, but not limited to”. The term“connected” means “directly or indirectly connected”, and the term“coupled” means “directly or indirectly connected”.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

When using only fringe-capacitance between metal lines within a givenmetal layer in constructing metal-to-metal capacitors on an integratedcircuit (IC), the ability for matching of unit capacitors may besuperior to the matching of unit capacitors whose configuration alsoincludes parallel-plate-capacitance. Likewise, higher capacitivedensities may be achieved with capacitors configured using solelyfringe-capacitance than with capacitors that also compriseparallel-plate structures. In order to maximize fringe-capacitance, themetal lines or strips used for top and bottom plates of the capacitormay be interdigitated with minimum spacing. Furthermore, by stacking topplate traces on top of each other and bottom plate traces on top of eachother, the parallel—or layer to layer—capacitance may be minimized.Thus, a top level layer in a multi-layer process may be used forrouting, and all layers below the top level layer, including the bottomlayer, may be configured as interdigitated structures to maximizecapacitance density.

Turning now to FIG. 1, one example of the interdigitated structure oftop and bottom plates for a metal-to-metal capacitor 100 usingfringe-capacitance is shown. More specifically, the capacitance isillustrated by the electric field lines from top capacitor plates102-108 to bottom capacitor plates 110-116, respectively. Parallel platecapacitance may be minimized and/or eliminated by stacking top plates102 and 106, 104 and 108, and bottom plates 110 and 114, 112 and 116 ontop of each other, respectively. By way of example, two metal layers, afirst metal layer 120 and a second metal layer 122, are shown in FIG. 1.Those skilled in the art will appreciate that depending on thefabrication technology, more than two metal layers may be available forconstructing metal-to-metal capacitors, and while for the sake ofsimplicity additional metal layers are not shown, the use of additionalmetal layers is possible and is contemplated. Note also that capacitor100 may comprise more (or less) than the four metal lines per layershown, and that the structure of any integrated circuit comprisingcapacitor structure 100 may extend beyond what is shown in FIG. 1, andsuch integrated circuit may also comprise components (not shown) inaddition to capacitor 100. However, for the sake of simplicity, only thestructure of capacitor 100 is shown in FIG. 1 (as well as in subsequentFIGS. 2-3.) Each plate shown in FIG. 1 may represent a metal trace orstrip within the respective metal layer in which it is configured. Asshown, top plates 102-104 and bottom plates 110-112 may be metal stripsin metal layer 120, and top plates 106-108 and bottom plates 114-116 maybe metal strips in metal layer 122. The electric field shown between topand bottom plates 102-108, and 110-116, respectively, may represent thecapacitance of a metal-to-metal capacitor formed using metal strips102-116. Also, in various embodiments of capacitors configured accordingto principles of the present invention, metal layer 122 may in fact be abottom metal layer, as will be further discussed below.

FIG. 2 shows one example in which metal layer 122 may be a bottom metallayer 220 of a capacitor 200, comprising the interdigitated structure oftop capacitor plates 202-204 and bottom capacitor plates 206-208. Noteagain that the four metal lines within bottom metal layer 220 are shownby way of example, and that capacitor 200 may comprise more than fourmetal lines or strips within bottom metal layer 220, as well asadditional metal layers similarly configured on top of bottom metallayer 220. As seen in FIG. 2, one possible drawback to using bottommetal layer 220 in configuring metal-to-metal capacitors is thecapacitance that may develop from top plates 202-204 and bottom plates206-208 to substrate 210. The undesirable capacitance is illustrated inFIG. 2 via the electric field lines from top capacitor plates 202-204and bottom capacitor plates 206-208 to substrate 210. While thecapacitance developed between bottom plates 206-208 and substrate 210may be tolerable, the capacitance developed between top plates 202-204and substrate 210 may be highly undesirable due to possible chargebleed-off when the voltage on top plates 202-204 is varied.

In order to minimize the capacitance developed between top plates202-204 and substrate 210, a low-impedance (finite resistance)conductive plate constructed in a polysilicon layer may be configuredbetween metal layer 220 and substrate 210. This is illustrated in FIG.3. Polysilicon plate 312 may be inserted between metal layer 220 andsubstrate 210, and tied to bottom plates 206-208, thereby providing ashield for capacitance developed from top plates 202-204 to substrate210. In alternate embodiments, polysilicon plate 312 may comprise anumber of strips instead of a single plate. The configuration shown inFIG. 3 may however result in a large capacitance from bottom plates206-208 to substrate 210, and a parallel top-plate-to-bottom-platecapacitance by virtue of bottom plates 206-208 being tied to polysiliconplate 312. This may be undesirable due to possible field-oxide thicknessvariation over the surface of a wafer and between fabrication lots.

One possible way to overcome these problems may be to bootstrappolysilicon plate 312 to top plates 202-204, instead of tyingpolysilicon plate 312 to bottom plates 206-208. FIG. 4 shows oneembodiment of a bootstrapping circuit which is configured to couplepolysilicon plate 312 to top capacitor plates 202-204, driving thecapacitance developed from polysilicon plate 312 to substrate 210,resulting in polysilicon plate 312 moving identically to the voltagelevel of top plates 202-204. The configuration shown in FIG. 4 mayresult in eliminating charge transfers that may take place from topplates 202-204 to polysilicon plate 312, and while a capacitance frombottom plates 206-208 to polysilicon plate 312 may exist, thecapacitance developed from top plates 202-204 to bottom plates 206-208may comprise solely fringe-capacitance. Referring back to FIG. 3, inalternate embodiments, polysilicon plate 312 may be replaced with adiffusion layer—for example an n-well diffusion layer—configured withinsubstrate 210, and bootstrapped to top capacitor plates 202-204 in amanner similar to that shown in FIG. 4 for polysilicon plate 312.

As shown in FIG. 4, polysilicon plate 312 is represented by node 420,the top-plate-to-polysilicon capacitance is represented by capacitor408, the bottom-plate-to-polysilicon capacitance is represented bycapacitor 412, the top-plate-to-bottom-plate capacitance is representedby capacitor 410, and the polysilicon-to-substrate capacitance isrepresented by capacitor 406. Terminal 422 represents top plates 202-204and terminal 424 represents bottom plates 206-208. Top plate terminal422 may be coupled to the gate of NMOS device 402, resulting in node 420tracking terminal 422, and no considerable voltage change acrosscapacitor 408 (i.e. no considerable current flowing from top plateterminal 422 to polysilicon plate node 420). This may effectively removecapacitor 408 from the circuit during circuit operation, which,referring again to FIG. 3, would functionally eliminate the parasiticcapacitance from top plates 202-204 to polysilicon 312, though acapacitance between bottom plates 206-208 and polysilicon 312 may stillexist. Since there is no parasitic capacitance from bottom plates206-208 (terminal 424 in FIG. 4) to substrate 210 (terminal 426 in FIG.4), all available metal layers, including bottom metal layer 200, may beused to form the desired metal-to-metal capacitors, with onlyfringe-capacitance forming from top plates 202-204 to bottom plates206-208, respectively. It should be noted that while the bootstrappingcircuit in FIG. 4 is shown being implemented with an NMOS device, use ofother devices and/or circuits which may facilitate reducing and/oreliminating charge transfer from top plate node 422 to polysilicon platenode 420 is possible, and is contemplated.

FIG. 5 shows one embodiment of a switched capacitor circuit 500 that maybe used in a delta-sigma analog to digital converter (ADC). Circuit 500shown in FIG. 5 may be configured with an amplifier 502—which may be anoperational transconductance amplifier—, input capacitors 506 and 508,feedback capacitors 510 and 512, capacitors 504 and 514, and switches516-522. Capacitors 504-514 may be metal-to-metal capacitors configuredon the integrated circuit that comprises switched capacitor circuit 500.Applying the bootstrapping configuration shown in FIG. 4 to the inputsof amplifier 502 for capacitors 504-514 may result in more accuratematching of capacitors 504-514, and consequently in a more accurateswitched capacitor circuit 500.

FIG. 6 illustrates how capacitors 506 and 508 may be bootstrappedthrough their respective top plates to differential inputs of amplifier502, according to the bootstrapping configuration shown in FIG. 4. Thetop plate of capacitor 506 may be coupled to differential input terminalInput+ of amplifier 502, and the top plate of capacitor 508 may becoupled to differential input terminal Input+ of amplifier 502. Itshould be noted that while capacitors 406, 408 and 412 (shown in FIGS. 4and 6) represent the various parasitic capacitances as previouslydescribed and illustrated in FIGS. 1-3, capacitor 410—that is, thecapacitance developed between the top and bottom plates—represents theactual desired capacitance of metal-to-metal capacitors 506 and 508.Hence, capacitors 506 and 508 in FIG. 6 may each represent thestructural equivalent of capacitance 410 shown in FIG. 4. In otherwords, capacitors 506 and 508 may be metal-to-metal capacitors, with therespective value of each capacitor corresponding to capacitance 410 fromFIG. 4. The differential input stage of amplifier 502 may comprise PMOSdevices 608 and 610, and NMOS device 402 may be configured to follow thecommon mode input of amplifier 502, driving the polysilicon platewithout affecting the performance of amplifier 502, or the capacitanceat the top plate. While the bootstrapping configuration is only shownfor capacitors 506 and 508, capacitors 504 and 510-514 may also bebootstrapped in a similar manner, with the top plate of each capacitorfacing respective input nodes, Input+ or Input−, of amplifier 502. Ineach instance, the top-plate-to-bottom-plate capacitance (410 in FIG. 4)may correspond to the actual capacitance. For example, the top plate ofcapacitor 510 may be coupled to switch 518, with switch 518 coupledbetween top plate 422 and Input+ of amplifier 502.

Although the embodiments above have been described in considerabledetail, other versions are possible. Numerous variations andmodifications will become apparent to those skilled in the art once theabove disclosure is fully appreciated. It is intended that the followingclaims be interpreted to embrace all such variations and modifications.Note the section headings used herein are for organizational purposesonly and are not meant to limit the description provided herein or theclaims attached hereto.

1. A capacitor on a semiconductor having accurate, high densitycapacitance between a first node corresponding to a top plate of thecapacitor and a second node corresponding to a bottom plate of thecapacitor, the capacitor comprising: one or more layers of conductivestrips forming the capacitor, wherein for each pair of neighboringconductive strips within each of the one or more layers of conductivestrips: a first conductive strip of the pair of neighboring conductivestrips is coupled to the first node and not to the second node; and asecond conductive strip of the pair of neighboring conductive strips iscoupled to the second node and not to the first node; and alow-impedance conductive layer configured beneath a bottom layer of theone or more layers of conductive strips, and spanning an area underneathconductive strips coupled to the first node and conductive stripscoupled to the second node, wherein the low-impedance conductive layeris electrically coupled to the first node and not to the second node toreduce charge transfers from the first node to the low-impedanceconductive layer.
 2. The capacitor of claim 1, wherein the low-impedanceconductive layer is coupled to the first node to reduce parasiticcapacitance from the second node to a substrate of the semiconductor inaddition to reducing charge transfers from the first node to thelow-impedance conductive plate.
 3. The capacitor of claim 1, wherein theone or more layers of conductive strips are aligned so that conductivestrips coupled to the first node lie above other conductive stripscoupled to the first node.
 4. The capacitor of claim 3, wherein the oneor more layers of conductive strips are aligned so that conductivestrips coupled to the second node lie above other conductive stripscoupled to the second node.
 5. The capacitor structure of claim 1,wherein the low-impedance conductive layer is a low-impedance conductiveplate.
 6. The capacitor of claim 5, wherein the low-impedance conductivelayer comprises one of: a solid planar low-impedance conductivematerial; and a plurality of low-impedance conductive strips.
 7. Asemiconductor device comprising a capacitor, the semiconductor devicecomprising: one or more layers of conductive strips forming thecapacitor, wherein for each pair of neighboring conductive strips withineach of the one or more layers of conductive strips: a first conductivestrip of the pair of neighboring conductive strips is coupled to thefirst node and not to the second node; and a second conductive strip ofthe pair of neighboring conductive strips is coupled to the second nodeand not to the first node; and a low-impedance conductive layerconfigured beneath a bottom layer of the one or more layers ofconductive strips, and spanning an area underneath conductive stripscoupled to the first node and conductive strips coupled to the secondnode, wherein the low-impedance conductive layer is electrically coupledto the first node and not to the second node to reduce charge transfersfrom the first node to the low-impedance conductive layer; wherein thecapacitor has an accurate, high density capacitance between a first nodecorresponding to a top plate of the capacitor and a second nodecorresponding to a bottom plate of the capacitor.